Field of the Invention
The invention relates to a method for detecting the rotational speed of electronically commutated fans, in which the rotational speed is indicated by a clock signal having a corresponding number of clock pulses per unit time. The invention also relates to a circuit configuration for detecting the rotational speed of electronically commutated fans, including a device for producing a clock signal with a corresponding number of clock pulses per unit time corresponding to the rotational speed of the fan.
Relatively powerful power supply units need electronically commutated fans to cool them. Electronically commutated fans cause current fluctuations on their power supply lines, which vary from fan to fan and not only from fan type to fan type.
In order to monitor and to control the fans, the rotational speed of the fans has to be detected electronically to derive a clock signal therefrom having an appropriate or corresponding number of clock pulses per unit time.
It is known for special fans to be used which pass an internal clock signal to the exterior from the electronic commutation through an additional line as a clock signal as mentioned above (see Papst Catalog, Equipment Fans, 94/95, Papst-Motoren GmbH & Co KG, Karl-Maier-Stra.beta.e 1, D-78112 St. Georgen/Schwarzwald, P.O. Box 1435, in the Chapter entitled "Equipment fans for DC voltage" and "Variants"). The disadvantage thereof is that such special fans are expensive.
It is also known for the current fluctuations which are caused by a fan to be detected by a current measurement resistor, to be filtered out through a high-pass filter, and then to be supplied to a pulse former in order to form the clock pulses of the clock signal mentioned above (German Published, Non-Prosecuted patent application 26 17 131, corresponding to U.S. Pat. No. 4,097,789). The disadvantage thereof is that the level and the form of the current fluctuations which are caused by the fan on its power supply lines differ to such an extent from fan to fan that matching to each fan is required.
Furthermore, German Published, Non-Prosecuted Patent Application DE 196 01 040 A1 discloses a method and a circuit configuration for detecting the rotational speed of electronically commutated fans, in which the rotational speed is indicated by a clock signal with an appropriate number of clock pulses per unit time, and a DE Company Document: "Motor Control Seminar 1986", page E15, SGS Halbeiter Bauelemente GmbH, April 1986, discloses the use of maximum current gradients for measuring current fluctuations which are caused by fans on the power supply lines as a function of the rotational speed, and deriving clock signals for rotational speed information therefrom.